Transformer insulating end ring

ABSTRACT

A transformer end ring is formed from laminated sheets, at least some of which are formed from a rigid synthetic plastic material reinforced with fibers of inorganic insulating material disposed all around the central opening and forming at least one group of fibers in each sheet, the sheets being bonded together by the plastic. Substantially all of the fibers in each group in the end ring are substantially parallel to one another, and cross fibers in the adjoining sheets.

United States Patent 1111 3,839,133

Heighes Oct. 1, 1974 [54] TRANSFORMER INSULATING END RING 2,844,354 7/1958 Warnken 161/42 X 3,467,932 9/1969 Feather 336/197 [75] Inventor: John Heighes, ChurChdOWn, 3,750,070 7/1973 Rissinger 336/197 X England [73] Assignee: Permali, Incorporated, Mount Primary ExaminerPhilip Dier Pleasant, Pa. Attorney, Agent, or Firm-Brown, Murray, Flick & 22 Filed: July 9, 1973 l [21] Appl. No.: 377,384 [57] ABSTRACT A transformer end ring is formed from laminated [52] Cl 161/42 7 6 sheets, at least some of which are formed from a rigid synthetic plastic material reinforced with fibers of in- [51] B32! 1/04 H0lf27/30 organic insulating material disposed all around the central opening and forming at least one group of fi- [58] Fleld of Search 161/48 bers in each sheet, the sheets being bonded together 336/197 by the plastic. Substantially all of the fibers in each group in the end ring are substantially parallel to one [56] References Clted another, and cross fibers in the adjoining sheets. UNITED STATES PATENTS 2,177,484 10/1939 Fruth .L 161/42 x 7 Clams 9 D'awmg F'gures w mw Q Q E a =5 J E E E EILEHI/ALI *IJ .T AHHWFLILLWIMRIPITAHHLILW Z EAW P1 2 PATENTED 1 74 SHEET 10F 3 PATENMOBI H 3.839.133

sneer ear 3 TRANSFORMER INSULATING END RING This invention relates to insulating end rings for transformers of the type having a magnetic core provided with a leg that has a yoke at each end, with a coil or winding surrounding the leg. In such transformers it is necessary to provide means for locating the coil endwise and, in particular, for restraining the coil against endwise movement when subjected to the very heavy endwise-forces that may occur under overload or short circuit conditions.

The construction of efficient insulating end rings for such transformers in an economical manner presents many difficulties. The end rings must have good insulating properties and also considerable mechanical strength. They should be designed to resist the peculiar bending stresses experienced in practice and resulting from the normal constructional arrangement, in which the anchorages for an end ring are located at a limited number of points adjacent to the transformer yoke that normally extends diametrically across the end ring.

Transformer end rings have previously been manufactured of steel or other metal with insulating pads, and also of composite synthetic plastic, but existing constructions suffer from various disadvantages. For example, the limited mechanical strength of some previous end rings has required the use of rings of appreciable thickness in a direction perpendicular to the plane of the ring, thus occupying valuable space and in some cases increasing the overall dimensions of the transformer. Some existing end rings have also suffered from thermal instability, due to the fact that the rate of heat generation within the ring material is higher than the rate of heat loss from the surface of the material, and the rate of heat generation increases with increasing temperature so that a condition of thermal runaway can be encountered.

Accordingly, it is an object of the present invention to provide an improved construction of a transformer insulating end ring which will solve or alleviate at least some of the problems experienced with existing end rings.

Broadly stated, the invention relates to an insulating end ring for an electrical transformer, the ring having a laminated composition with the planes of the laminations lying parallel to the general plane of the ring. At least some of the laminations are of fiber-reinforced plastic and each such lamination comprises one or more co-planar sections, with each section, or the complete lamination, formed of a rigid synthetic plastics material combined with a plurality of reinforcing strands or fibers of an inorganic insulating material.

According to a preferred feature of the invention, the

- fibers all extend parallel with one another within each section or lamination. Where a lamination is made up of a plurality of co-planar sections, the group of parallel fibers in each section is at an angle to the groups of parallel fibers in the adjoining sections. Also, each lamination is rotated on its axis relative to the adjoining laminations from a few degrees to as much as 90 so that the fibers in each lamination will cross those in the adjoining laminations.

Moreover it is preferred that all the strands or fibers in each complete lamination should be parallel or unidirectional so that the laminations can be made by a particularly simple and convenient manufacturing process. The mechanical strength of the ring is maintained at the required level by ensuring that the strands or fibers in adjoining laminations are perpendicular or inclined.

Alternatively, each of the laminations may comprise two or more co-planar sections, with the strands or fibers of each section arranged in a direction inclined to the direction of the strands or fibers in adjoining sections within the same lamination. In such case it will usually be arranged that the direction of the strands or fibers in each section is approximately parallel to a tangent to the adjacent part of the profile of the ring.

In any case the synthetic plastic and the reinforcing strands or fibers are preferably resistant to temperatures of up to C., and preferably up to at least C, and according to another preferred feature of the invention the composite reinforced synthetic plastic has a substantially flat, or negative, tanS/temperature coefficient over the the greater part of the designed temperature range, for example up to 60C., with an operating range between 20 and 100C.

The reinforcing strands or fibers are preferably formed of a non-hygroscopic material such as glass, and the synthetic plastic is preferably an epoxy resin or other thermo-setting synthetic plastic; for example, phenol formaldehyde or a silicone resin or a polyester resin.

In preferred methods of constructing end rings according to this invention the laminations, or lamination sections, are formed separately, laid up in a dry partlycured state, and then finally cured under heat and pressure, the pressure being applied across the planes of the laminations.

The invention also consists in a transformer having a core, including a leg with a yoke at each end thereof, an electrical coil or winding surrounding the leg, an end ring as defined located at one or both ends of the coil, and anchorage means for locating the end ring lengthwise of the core leg.

The invention may be performed in various ways and one specific embodiment with various possible modifications will now be described by way of example and with reference to the accompanying drawings, in which FIG. 1 is a side view of a three-coil transformer;

FIG. 2 is an end view;

FIG; 3 is a plan view;

FIG. 4 is an enlarged fragmentary side view of one of the end rings;

FIG. 5 is a plan view of an end ring illustrating diagrammatically the parallel fibers and with one of the laminations broken away to show part of the lamination beneath it;

- FIGS. 6 and 7 are views similar to FIG. 5 of two different modifications of the invention;

FIG. 8 is a diagram illustrating the relationship between temperature and tan 8 coefficient; and

FIG. 9 is another diagram illustrating the crossbreaking strength of a composite laminated material in a typical end ring constructed according to the invention.

Referring to FIGS. 1, 2 and 3 of the drawings, a transformer has a laminated magnetic core provided in this case with three spaced parallel legs I joined at top and bottom to parallel horizontal yokes 2. Engaging the opposite sides of the lower yoke is a pair of metal channels 3 clamped tightly against the yoke by bolts 4. A similar pair of channels 6 are clamped against the opposite sides of the upper yoke by bolts 7. A transformer coil 8 encircles each leg of the core and rests on an insulating end ring 9 supported by the lower yoke and channels. The portions of these rings that project laterally from the channels are supported on blocks, usually non-metallic, that rest on the tank bottom or a projection therefrom. Mounted on top of the coils are upper insulating end rings 13 that are spaced below the upper yoke and the upper channels. Engaging each of these rings at circumferentially spaced points 90 apart are jack screws 14 threaded in lugs 15 projecting from the outer sides of the upper channels. These screws are screwed down tightly against the upper end rings so that when the coils are subjected to heavy overload or short circuit conditions the tendency of the coils to move upwardly along the cores legs is resisted by the upper end rings that are held rigidly in place by the upper jack screws. The end rings also provide electrical insulation between the ends of the transformer coils and the yokes of the magnetic core. It will be noted that the end rings are located in the alternating magnetic field of the transformer.

In accordance with this invention, each of the end rings is constructed as a composite laminated structure such as indicated in FIG. 4. The end ring shown in FIG. 5 is circular, but it could be square or some other shape. Each lamination of the ring is formed from a large number or group of fibers 21 of inorganic insulating material such as glass, laid uni-directionally; i.e., parallel with one another, all around the central opening in the lamination. The fibers are bonded together by a rigid thermosetting plastic 22, such as epoxy resin, which also adheres strongly to the individual fibers. The fibers in each lamination cross the fibers in the immediately adjoining laminations. They may cross at an oblique angle or, as shown in FIG. 5, at right angles. In the ring shown in FIG. 5 alternate laminations have their fibers extending in the same direction, while the fibers of the rest of the laminations extend at right angles to those first mentioned. If they were to extend at 45, for example, the fibers in the top lamination might extend north-south, those in the next lamination northeast-northwest, those in the next east-west, and those in the next southeast-southwest, etc.

The preferred method of making such an insulating end ring is to produce a thin strip consisting of reinforcing fibers or strands extending lengthwise of the strip and coated or impregnated with a liquid epoxy resin. The strip then is dried and partly cured in a heating chamber and then cut into sheets of the desired size and shape for end rings. These sheets are then stacked, with each sheet rotated or oriented relative to the adjoining sheets so that their fibers cross. Finally, the stack of sheets or laminations is cured and bonding is completed under high temperature and pressure.

In the modification shown in FIG. 6 each sheet or lamination 25 of the end ring is formed from several separate pie-shape sections 26 joined side by side, with the reinforcing fibers 27 in each section parallel to the tangent to the curved outer edge of that section. The fibers in each section therefore will meet the fibers in the adjoining sections at an oblique angle, and the fibers as a whole in the lamination will extend around its central opening. The different laminations are so oriented that the radial joints between their sections are located half way between the joints between the sections in the adjoining laminations. Therefore, the fibers in each lamination cross the fibers in the adjoining lam inations.

In the further modification illustrated in FIG. 7, each sheet or lamination 30 of the end ring is shown built up from four separate sections 31 joined edge to edge. Again, all of the reinforcing fibers 32 within each of these sections are substantially parallel to one another. Each section has a straight edge perpendicular to the parallel fibers therein and bonded to a straight edge of an adjoining section. However, this last-mentioned straight edge is substantially parallel to the fibers in that adjoining section and may be connected by an arcuate edge 33 with the inner end of the other straight edge of the same section so that all of the arcuate edges together will form the wall of a circular central opening in the lamination. The outer edge of the lamination may be square or circular. Each lamination is rotated up to relative to the adjoining laminations, FIG. 7 showing the angle to be 90.

It will be noted that the end rings of both FIGS. 6 and 7 provide good crossbreaking strength or resistance to bending stress in respect of the four pressure points afforded by the jack screws that engage the rings when they are installed in a transformer.

Typical insulating end rings made in accordance with this invention may have external diameters of 40 inches or more, internal diameters of 18 inches or more and a thickness of between 1 and 4 inches.

Tests of a typical transformer end ring constructed according to the invention have shown that the tan 8 or power-loss co-efficient of the material varies with temperature as shown in the graph of FIG. 8. In a typical example the normal operating range of the transformer end rings may be between about 40 and 60 and, when subject to overload, the temperature may rise to a value of about C. Over this range the characteristic curve is substantially flat and actually has a negative gradient over the greater part of the range, and under these conditions the thermal stability is positive. In other words, assuming that the rate of heat loss from the end ring is at least equal to the rate of internal heat generation at any specified temperature, any rise in temperature will not increase the rate of heat generation and may actually result in a reduction, whereas the heat loss may be expected to rise at the increased temperature.

FIG. 9 illustrates the crossbreaking strength of an insulating end ring manufactured in accordance with the invention, having a thickness of between 1.5 inches and 2 inches, compared with a similar specimen formed of a material such as densified wood. It will be seen that at an assumed span of 30 inches the end ring has a crossbreaking strength in excess of 80 lbs./sq/ in. X 10 The composite laminated material of the end rings described is temperature resistant up to temperatures of at least C., and the material is naturally nonhygroscopic since both the reinforcing fibers and the synthetic plastic epoxy resin, have very low water absorption properties. The weight of the end ring is also relatively small by comparison with some prior constructions, the specific gravity of the described material being 2.I. Although this may be slightly greater than some types of laminated plastic, the strength of the end ring constructed as described is such that the thickness can be reduced and weight saved, whereby the physical dimensions of the ring and so the overall dimensions of the transformer itself can be reduced.

Another important characteristic of an end ring in a transformer is its ability to resist internal discharge under the high voltage electrical fields experienced, and particularly its discharge inception/extinction level. Tests have indicated that the level for a typical end ring constructed in accordance with the invention is considerably higher than with many other types of composite synthetic plastic.

A particular advantage of laminated insulating end rings constructed in accordance with the invention is that it is possible to manufacture such rings to any required dimensions with a minimum of difficulty. Since the end rings are built up in a dry layup process, different dimensions can be produced simply by changing the sizes of the sheets used in the layup stack, and varying the number of sheets in the stack to adjust the thickness of the final end rings. By comparison, in any process involving the winding of a continuous strand or fiber to form a ring, such as disclosed in U.S. Pat. No. 3,467,932, it is necessary to provide a specially shaped mandrel or former for each particular size of ring required.

By making use of uni-directional fibers as disclosed herein instead of a cloth made from them, the cost is reduced. Also, since uni-directional fibers can lie straight as distinguished from woven fibers that are wavy due to their crossing one another, the strength of the end rings is greater.

It is also possible in some examples of the invention for the end rings to include an intermediate layer or layers of a different material, or of the same material orientated in a different manner, and sandwiched between fiber reinforced laminations as described. The intermediate layer can be relatively light and is not required to have the same tensile strength and crossbreaking strength, its main function being to act as a spacer or compression member, and also of course as an electric insulator.

Suitable materials for the intermediate layer or layers are wood veneers impregnated with phenol formaldehyde or transformer oil, or woven or felted fibrous material or other sheet material such as paper, all impregnated with a thermo-setting resin such as a phenolic resin. Alternatively, the intermediate layer may be of the same material as the main reinforced laminations, but laid on edge"with the planes of their laminations perpendicular to the planes of the main laminations.

According to the provisions of the patent statutes, l have explained the principle of my invention and have illustrated and described what I now consider to represent its best embodiment. However, I desire to have it understood that. within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

I claim:

ll. A transformer insulating end ring comprising laminated sheets provided with a central opening there through for a leg of a magnetic core, at least some of said sheets being a rigid synthetic plastic reinforced with fibers of inorganic insulating material, said fibers located all around said opening in at least one group in each sheet, substantially all of the fibers in each group in said laminated sheets being substantially parallel to one another and crossing fibers in the adjoining sheets, and said plastic bonding said sheets together into a rigid insulating ring.

2. A transformer insulating end ring according to claim l, in which all of the fibers in each sheet are substantially parallel.

3. A transformer insulating end ring according to claim 1, in which each of said sheets is circular in the form of a plurality of pie-shape sections, and the fibers in each of said sections are substantially parallel to a tangent to that section, whereby the fibers in the sheet extend around its central opening.

4. A transformer insulating end ring according to claim 1, in which each of said sheets is composed of a plurality of sections joined edge to edge, each section having a straight edge perpendicular to the parallel fibers therein and joined to a straight edge of an adjoining section, and said last-mentioned straight edge being substantially parallel to the fibers in said adjoining section.

5. A transformer insulating end ring according to claim 1, in which an end ring between 1.5 inches and 2 inches thick has a crossbreaking strength in excess of lbs. per square inch X 10 over a 30 inch span.

6. A transformer insulating end ring according to claim 1, in which said fibers are resistant to temperatures between C and C.

7. A transformer insulating end ring according to claim 1, having positive thermal stability over its operating range.

' UNETED STATES PATENT @FHCE CERT-EWCATE @1 @QRRECTEUN Patent No. 3,839,133 Dated October 19 1974 fiwentofls) @Tohn Heighes It is certified that error-appears in the above-identified patent and that said Letters Patent are hereby correcteci as shown below:

Claims priority prsvisional application Great Britain, October 25, 1972, LL9 O9LL/72 si ned and sealed this 18th day of Februar 1975.

(SEAL) Attest:

- C, MARSHALL DANN RUTH C3a MASON Commissioner of Patents Attesting Officer and Trademarks was STATES MTENT @FFEQE @ERTEFECAT EGTEQN Dated October l, 197

Patent No. 3, 839,133

fi wentorw) ifohn Heighes I It is certified that error-appears in the above-identifiedpatent and that said Letters Patentare hereby eorrected as shown below:

Claims priority provisional application Great Britain, October 25, 1972, 49,09Lg/72 Signed and sealed this 18th day of Februar 19756 (SEAL) Attest:

- (3 MARSHALL DANN RUTH C MASON Commissioner of Patents Attesting Officer and Trademarks 

1. A transformer insulating end ring comprising laminated sheets provided with a central opening therethrough for a leg of a magnetic core, at least some of said sheets being a rigid synthetic plastic reinforced with fibers of inorganic insulating material, said fibers located all around said opening in at least one group in each sheet, substantially all of the fibers in each group in said laminated sheets being substantially parallel to one another and crossing fibers in the adjoining sheets, and said plastic bonding said sheets together into a rigid insulating ring.
 2. A transformer insulating end ring according to claim 1, in which all of the fibers in each sheet are substantially parallel.
 3. A transformer insulating end ring according to claim 1, in which each of said sheets is circular in the form of a plurality of pie-shape sections, and the fibers in each of said sections are substantially parallel to a tangent to that section, whereby the fibers in the sheet extend around its central opening.
 4. A transformer insulating end ring according to claim 1, in which each of said sheets is composed of a plurality of sections joined edge to edge, each section having a straight edge perpendicular to the parallel fibers therein and joined to a straight edge of an adjoining section, and said last-mentioned straight edge being substantially parallel to the fibers in said adjoining section.
 5. A transformer insulating end ring according to claim 1, in which an end ring between 1.5 inches and 2 inches thick has a crossbreaking strength in excess of 80 lbs. per square inch X 103 over a 30 inch span.
 6. A transformer insulating end ring according to claim 1, in which said fibers are resistant to temperatures between 100*C and 150*C.
 7. A transformer insulating end ring according to claim 1, having positive thermal stability over its operating range. 